Hello,
This really depends on what you're trying to accomplish. When you setup a custom Incoming/Outgoing Mail Policy, then what this means is that any specific settings tied to that policy (IE: Anti-Spam/Anti-Virus/Filters/DLP/ETC) will be tied to that Sender/Recipient.
You can also review further info here : ASyncOS ESA Mail Policies
Thanks!
-Dennis M.